1. Technical Field
The present invention relates to the structure of a metal-oxide-semiconductor field-effect transistor (MOSFET) sensor and, more particularly, to the structure of a MOSFET sensor for use in biomedical sensing.
2. Description of Related Art
Referring to FIGS. 1A and 1B, an ion-sensitive field-effect transistor (ISFET) 100 must be provided with a reference electrode 11 in order to sense hydrogen ions or other selective ions, wherein the reference electrode 11 serves to provide the reference voltage needed by the gate of the ISFET 100. Once the surface of a sensing membrane 10 adsorbs the ions to be sensed, the original gate voltage varies such that the output current is changed. Thus, the pH value of a solution can be sensed.
As shown in FIG. 1A, when the reference electrode 11 is not integrated with the ISFET 100, the relatively long distance between the reference electrode 11 and the sensing membrane 10 gives rise to a relatively weak electric field therebetween, given the same electric potential difference. In addition, as the reference electrode 11 and the sensing membrane 10 form a parallel pair of electrodes with no gradient in the electric field therebetween, the resultant ion migration is relatively insignificant. Therefore, when the reference electrode 11 is not integrated with the ISFET 100, the position of the reference electrode 11 need not be particularly designed.
However, with the current trend toward sensor miniaturization and high integration, the conventional arrangement which requires the ISFET 100 to be connected to an external reference electrode 11 has caused inconvenience in use and prevented further downsizing. Consequently, it is difficult to integrate both the ISFET 100 and the reference electrode 11 into a micro-current system, and applications of the ISFET 100 and the reference electrode 11 are subjected to limitations.
FIG. 1B shows a prior art structure in which the reference electrode 11 and the ISFET 100 are integrated into a single chip. After integration, the reference electrode 11 and the sensing membrane 10 form a coplanar electrode structure, and yet electric field distribution between the reference electrode 11 and the sensing membrane 10 is rendered non-uniform. Moreover, due to cost and miniaturization considerations, the reference electrode 11 must be very close to the sensing membrane 10 such that the electric field therebetween is substantially increased and causes relatively significant ion migration. Such ion migration, nevertheless, tends to disturb response signals and thus impair the stability of the ISFET sensor.